Okay it sounds really boring. and it pretty much is. BUT it is very important so i have to make a lesson about it...
In Spanish, most words have a feminine and a masculine form. The way you tell the difference is pretty easy. If it ends in an "o" it is masculine. If it ends in an "a" it is feminine. Words that don't end in either "o" or "a" probably won't change any. Some examples:
Mi perro es cafe (my dog is brown)-- You can tell that the dog is a boy because the word "perro" (meaning dog) ends in an "o". If you had a girl dog, it would be: "Mi perra es cafe" because the "o" in perro becomes an "a".
The same thing for multiple words in a sentence that end in "o" or "a":
Mi camisa es roja (my shirt is red)-- Since camisa ends in an "a" (because it is a feminine article of clothing) the color has to end in the same letter. It wouldn't make any sense if you said "Mi camisa es rojo" because the adjectives/nouns (camisa and rojo) aren't agreeing with each other-they are saying the shirt is two different genders.
For clothing, IT DOESN'T MATTER WHO IS WEARING IT!!!!!! If a guy is wearing a red shirt the sentence would still be, "Mi camisa es roja".
An example of when you don't change the last letter would be if you had a sentence that said, "Los arboles son verdes " (the trees are green). Since both of the adjectives/nouns (arobles and verdes) don't end in either an "o" or "a" they stay the same no matter what gender you want to make the trees (okay i know that sounds weird...).
The other part to this rule is plurals. As you can see above, both arboles and verdes end in "s" because the noun is plural (arboleS and verdeS).
Wow this is a long lesson.... Post questions/tries to get this rule correct. Whoever gets the longest agreeable sentence gets a gold star (oooo sparkly

lol)